At 9lbs, my guy is already starting to fit into his OS. He can still wear the better-fit infant size prefolds so I do think those are worth it (especially since you can use them to stuff pockets, use as burp cloths or whatever after they are outgrown).

I had a 7lb 10oz baby and we used newborn sized diapers for about 2 weeks until the cord stump fell off. If I had to do it again, I wouldn't have gotten newborn pockets or newborn AIOs.

We had intended to use disposables until DD fit into OS dipes. Then, I learned about rentals. We rented 24 NB simplex from KBTM and it was something around $30/mo. (plus deposit). It was definitely cheaper than disposables and was a great experience. I think her prices may have risen since we did the program, but I still recommend it!